Here's my set-up: I have an "@outlook.com" email address that has always synced beautifully across my Lumia 830, Dell XPS 12 (Standard Win8.1) and my Surface 2 (RT), the latter two with the built-in Mail app in Win8.1.

This past Saturday, I got 4 emails from Microsoft billing confirming some apps that I had bought a few weeks ago. This didn't seem in any way strange other than there was obviously a bit of a delay. The crazy thing was that I noticed on Tuesday that I wasn't getting any new emails on any of my devices which seemed strange.

I went onto Outlook.com, only to see that I had received about 15 emails in the meantime.

Trying to manually sync did nothing - it just said "up-to-date" - obviously wrong...

On the Lumia 830, I changed the sync settings from "as items arrive" to "every 15 minutes" and did a soft-reset. This got things moving again.

On the Surface 2 I tried a similar tactic, with no luck - nothing got the emails syncing again. I then uninstalled the Mail, People and Calendar app, reinstalled it, and everything works fine again.

I'm assuming this will work on the XPS 12, though I haven't actually done it yet..

So, I'm not so much looking for advice, as I seem to have found the solution. But I was wondering if anyone of you experts out there has any idea how something like this could have happened? It makes absolutely no sense to me...